Converting a Yamaha four stroke outboard from 9.9 to 15 hp

[onthejob]

I don't know if I'm chasing aliens or bigfoot, but I've always heard different rumors about being able to convert 9.9 outboards to 15 hp because they are essentially the same size, bore, pistols, part number, etc except for maybe a carb or jetting or throttle limiter, etc. Anybody ever successful at this? I'm doubting it because I'm sure eBay would've had a conversion kit ready to go by now unless Yamaha has suppressed them. I've got a 2005 Yamaha four stroke 9.9. I mean who wouldn't want an extra 5 hp if they could get it? My dingy could handle it and I just need it when loaded with four adults and gear to get up on a plane.

 

[juggernaut1]

Best thing to do is locate the parts list. If they are in the same "family" both the 9.9 and the 15 will be on the same parts list document but with distinguishing part numbers for certain components.

FWIW the 6hp and 8hp Yammy 2 stroke have different part numbers for the carby and exhaust with everything else being the same - the 8hp gets its extra hp by revving higher.

 

[northern]

I have a 20HP Merc I looked at changing to a 25HP. The dealer told me it was possible but I need to change the carb and lower unit gear ratio. The difference between a 20 and 25 is RPM in the engine and in the leg. Assume it is the same in a 9.9. It was less effort to purchase a 25 probably the same with a 15.

 

[onthejob]

So what you're saying is Bigfoot does exists? Yes I "heard" on my internet searches that the carb and exhaust are one of the main difference between the two. I just want to talk to someone that actually successfully converted one just with a carb and exhaust and maybe a throttle adjustment. I'm not much of an explorer. I like to ride on the wheel instead of inventing it.

Best thing to do is locate the parts list. If they are in the same "family" both the 9.9 and the 15 will be on the same parts list document but with distinguishing part numbers for certain components.

FWIW the 6hp and 8hp Yammy 2 stroke have different part numbers for the carby and exhaust with everything else being the same - the 8hp gets its extra hp by revving higher.

 

[onthejob]

From what I have seen and heard so far everything is the same except for the carb and maybe exhaust. The prop and lower looks to be the same. A throttle limiter may also involved. Rumor is they limited the 9.9 to get a 15 hp under 10 hp for certain lake restrictions. But I seriously don't know for sure. Thanks for input.

I have a 20HP Merc I looked at changing to a 25HP. The dealer told me it was possible but I need to change the carb and lower unit gear ratio. The difference between a 20 and 25 is RPM in the engine and in the leg. Assume it is the same in a 9.9. It was less effort to purchase a 25 probably the same with a 15.

 

[juggernaut1]

I'm not saying "big foot" does in fact exist - I'm giving you the methodology I used and also used by my local Yammy dealer when I enquired with them to do a 6hp to 8hp conversion.

Furthermore, if the 15 hp is the same cubic capacity as the 9.9 hp then it may not give you more mid-range to plane with more people - it may only give you more top end which is fruitless if the boat won't plane in the first place.

 

[Lazy Daze]

Although I've never done it (came close a few times, but never pulled the trigger), in some engines it can be as easy as intake/exhaust. I do not know on your Yammie. But, like Jugs mentioned, the first thing I would do is compare parts lists. You can do that at a place like: http://www.boats.net/parts/search/Yamaha/Outboard/parts.html Open up two windows, one for the 9.9 and one for the 15. I would start by comparing part numbers for big ticket items like cylinder heads. If those aren't the same, stop there.

 

[blaster]

Adding horsepower is expensive. Even a simple carb swap can get pricey. I would get a 15 then replace the decals with 9.9 and most won't know the difference. Like juggernaut said, you may only benefit with more top end from the increased fuel flow. If it's power from torque your after, get more displacement.

 

[Molson]

I know the 2 stroke merc's were a matter of a carb and exhaust tube, all other parts were the same between the 9.9 and 15.

 

[mystery]

Did anyone ever figure out if the 9.9 four stroke can be converted into 15 hp?

 

[Lazy Daze]

Did anyone ever figure out if the 9.9 four stroke can be converted into 15 hp?

As noted above... Compare the part numbers. When you do that, what do you find? It only takes a few minutes to do that.

 

[Blueone]

As noted above... Compare the part numbers. When you do that, what do you find? It only takes a few minutes to do that.

+1 ...It's the easiest quickest way... I did it with a 4hp Tohatsu converting it to a 6hp...carb and a prop were the only differences

 

[bajturner]

Converting a 9.9 four stroke to a 15 is not possible...not as a simple conversion, at least. They are different displacements with both bore and stroke differing.

The conversion you heard of is for 2-strokes which required pretty much just a carburation change.